With the recent advance of high velocity and large capacity optical fiber communication system, a high velocity optical modulator composed of an optical waveguide element, which is a typical example of an external modulator, has been widely available. Such an optical waveguide modulator is constructed as follows.
FIG. 1 is a cross sectional view schematically showing an optical waveguide element used in a conventional high velocity modulator. FIG. 2 is a cross sectional view schematically showing another optical waveguide element used in a conventional high velocity modulator.
An optical waveguide element 10 depicted in FIG. 1 includes a substrate 1 made of a Z-cut single crystal of electro-optical effect and a buffer layer 2 formed on the substrate 1. Moreover, the optical modulator 10 has a first and a second branched optical waveguides 3-1 and 3-2 in the substrate 1, formed by a titanium inter-diffusion method or the like. The optical waveguides 3-1 and 3-2 are made a pair to construct a Mach-Zehnder type optical waveguide.
Moreover, on the buffer layer 2, are provided a signal electrode to apply a modulating signal to an optical wave through the first branched optical waveguide 3-1, and ground electrodes 5-1 and 5-2. Then, in making the substrate 1 of a Z-cut single crystal like this case, the signal electrode 4 is disposed directly on the first optical waveguide 3-1. The ground electrodes 5-1 and 5-2 serve as opposed electrodes for the signal electrode 4, and are disposed as near as possible the signal electrode 4 so that the total impedance of the electrodes is matched to 50Ω of external impedance and the driving voltage is reduced.
An optical waveguide element 20 depicted in FIG. 2 includes a substrate 11 made of a X-cut single crystal of electro-optical effect and a buffer layer 12 formed on the substrate 11. Moreover, similar to the optical waveguide element 10, the optical waveguide element 20 has a first and a second branched optical waveguides 13-1 and 13-2 to construct a Mach-Zehnder type optical waveguide. Moreover, on the buffer layer 12, are provided signal electrode 14 and ground electrodes 15-1 and 15-2.
In this case, the branched optical waveguide 13-2 is disposed directly under the ground electrode 15-2 so as to apply chirp to an optical wave through the waveguide 13-2. The ground electrodes 15-1 and 15-2 serves as opposed electrodes for the signal electrode as mentioned above.
In the optical waveguide elements 10 and 20 shown in FIGS. 1 and 2, a given half-wavelength voltage is applied to the signal electrodes 4 and 14, to shift the phases of optical waves through the first branched optical waveguides 3-1 and 13-1 for the phases of optical waves through the second branched optical waveguide 3-2 and 13-2 by π, and thus, to switch on/off an optical signal superimposed on an optical waveguide.
The half-wavelength voltage is calculated on a voltage magnitude when the phase of the optical wave is shifted by π from a standard operation point, which is pre-determined on the optical intensity modulation curve of the optical waveguide element.
However, in such a conventional optical waveguide element as shown in FIGS. 1 and 2, the operation point may be shifted due to the change of environmental temperature. Therefore, the half-wavelength voltage is shifted from the pre-determined value, and thus, the optical waveguide element can not be switched on/off well.
Accordingly, in the case of employing the above-mentioned optical waveguide element for an optical modulator of high velocity and large capacity optical fiber communication system, the high reliability and the high stability of the fiber communication system can not be satisfied.